A few reasons:
1. For me, the damage is already done. Facebook has over a decade of data from me, and even if I were to delete my account the Cambridge Analytica story shows that Facebook has already shared that data. Even if there was a way for Facebook to delete all of my data, they've already lost control. It's like taking the bullets out of a persons gun after they have shot you. You're still wounded.
2. Facebook has largely become the social network where you interact with people you don't really care about. My feed is full of third-party content, and stuff about people I haven't seen in years - people I knew from school, people I don't even like, etc. If I delete Facebook I delete my ability to be nosy, and I think that's why a lot of people won't delete their accounts. Whether you're perving on the guy/girl at school that went from zero to ten, laughing about some asshole from school now has a shitty life and several failed relationships, or how many kids the girl from your English class has popped out since leaving school, Facebook will always be the platform for that.
3. Despite its flaws, it's the closest we've got to a universal social network. I do BJJ a few times a week, and outside of a few people I know that everyone at my gym is on Facebook, so if I want to chat to any of them I can use messenger, or I can add them as a friend.
Most of all, I genuinely don't believe anything bad will happen to Facebook as a result of this. Their higher-ups might get a grilling from ministers/congress, but they will get away with their crimes - and make no mistake, these should be serious crimes. Facebook allowed access to their data under false pretences, and the first news to come from the arrest warrant was that Facebook engineers were on-site right away at Cambridge Analytica. In a just world, Facebook would be fined for every breach of a persons data, to the tune of tens of billions. The fine should be so great that it forces Facebook to scale back their operations and completely restructure their business. In reality, someone might go to jail at Cambridge Analytica, but this practice will continue. If anything, this practice will be promoted as an unethical, but unpunishable tactic across the world.